D. Lawless, S. Kapoor, P. Kennepohl, D. Meisel and N. Serpone, J. Phys. Chem. 98 (1994), p. 9619; S. J. Oldenburg, R. D. Averitt, S. L. Westcort and N. J. Halas, Chem. Phys. Lett. 288 (1998), p. 243; Z. J. Jiang and C. Y. Liu, J. Phys. Chem. B 107 (2003), p. 12411; W. Wang and S. A. Asher, J. Am. Chem. Soc. 123 (2001), p. 12528; and D. Wang, V. Salgueiriño-Maceira, L. M. Liz-Marzán and F. Caruso, Adv. Mater. 14 (2002), p. 908 disclose methods by which metal nanoparticles are deposited to various inorganic supports. While these methods make it easy to handle metal nanoparticles and reduce metal nanoparticle agglomeration, the methods are very complex and are extremely difficult to control for repeatedly producing composite particles in a controlled and narrow nano-size.
Alternative routes for producing small composite particles have been suggested by others as a way to simplify the very complex reactions and control processes required by previously known processes of synthesizing such metal-inorganic composite particles. One method is sono-chemical deposition, as disclosed by V. G. Pol, A. Gedanken and J. Calderon-Moreno, Chem. Mater. 15 (2003), p. 1111. Another method is electroless plating, as disclosed by Y. Kobayashi, Y. Tadaki, D. Nagao and M. Konno, J. Colloid Interface Sci. 283 (2005), p. 601. Yet another method is electrostatic attraction techniques, as disclosed by J. Zhang, J. Liu, S. Wang, P. Zhan, Z. Wang and N. Ming, Adv. Funct. Mater. 14 (2004), p. 1089. However, these processes are still difficult to control, expensive, and/or not commercially scalable for mass production of nano-sized metal composite particles, which may be a desired range of size in order to dramatically improve catalyst efficiency and performance in commercial use of such composite particles.
Composite nano-sized particles have potential applications in various fields, such as surface-enhanced Raman scattering (SERS), as disclosed in S. Nie and S. R. Emory, Science 275 (1997), p. 1102, photonic crystals, as disclosed in Z. L. Wang, C. T. Chan, W. Y. Zhang, Z. Chen, N. B. Ming and P. Sheng, Phys. Rev. B 64 (2001), p. 113108, catalysis, as disclosed in C. W. Chen, T. Serizawa and M. Akashi, Chem. Mater. 11 (1999), p. 1381, and biochemistry for chemical sensors, as disclosed in S. A. Kalele, S. S. Ashtaputre, N. Y. Hebalkar, S. W. Gosavi, D. N. Deobagkar, D. D. Deobagkar and S. K. Kulkarni, Chem. Phys. Lett. 404 (2005), p. 136.
A polystyrene-metal nanocomposite particle was first disclosed in J. M. Lee, D. W. Kim, Y. H. Lee and S. G. Oh, Chem. Lett. 34 (2005) (7), p. 928. However, this process used already formed polystyrene spheres and deposited metal onto the surface of the spheres using a polyol process. The polyol process is a chemical reduction method using polyol, such as ethylene glycol and diethylene glycol, to chemically reduce a metal salt. In this process, polyol acts both as the solvent of the metallic precursor and as the reducing agent, such as disclosed in F. Fievet, J. P. Lagier, B. Blin, B. Beaudoin and M. Figlarz, Solid State Ionics 32/33 (1989), p. 198; P. Y. Silvert, R. Herrera-Urbina, N. Duvauchelle, V. Vijayakrishnan and K. Tekaia-Elhsissen, J. Mater. Chem. 6 (1996) (4), p. 573; and P. Y. Silvert, R. Herrera-Urbina and K. Tekaia-Elhsissen, J. Mater. Chem. 7 (1997) (2), p. 293. However, none of these references disclose any method of control that allows a one-pot process to produce nanoparticle clusters onto an inorganic particle surface, while controlling size of the composite and clusters.
Korean Patent Laid Open, entitled “Polymers and novel metals composites by alcohol reduction,” which was filed Apr. 22, 2004 in the Korean Patent Office to Seong Geun Oh (“Oh Reference”), teaches a method for reduction of metals, such as platinum, palladium, gold, silver, osmium, iridium, ruthenium, and rodium or mixtures of any of these and other elemental metals, to obtain nano-sized metal particles as a colloid phase. The application is incorporated herein by reference in its entirety.
The polyol process is one type of alcohol reduction process. In this process, it is preferred to use an alcohol with a high boiling point to reduce metals not easily reduced at lower temperatures. Some examples of alcohols are listed, including ethylene glycol, diethylene glycol, trimethylene glycol, and isopropylene glycol. The Oh Reference teaches several examples of metal reduction processes for precious metals such as silver, platinum, palladium, and gold. However, this reference fails to teach any process capable of producing metal-particle complexes. Instead, the process is used merely to prepare metal colloids.
In another application laid open, Korean unexamined patent application no. 10-200400039256, entitled “Manufacturing method of silver-sulfur-silica complex nanoparticles having antibacterial, antifungal properties,” which was published on May 10, 2004, particles were formed by binding silver ions on the surface of pre-existing 30-40 nanometer silica particles by adding the silica particles to a sulfur-containing functional additive, 3-mercaptopropyl trimethoxysilane, and using NaBH4 or ascorbic acid as a reducing agent. The reference teaches the precipitation of the particles of 30-40 nm using ethanol, followed by a pulverization process. The 3-mercaptopropyl trimethoxysilane functionalizes the surface of the silica particles, and the NaBH4 or ascorbic acid is used to reduce the silver ions from silver nitrate to elemental silver metal, which nucleates and grows on the functionalized surface of the silica particles. The resulting suspension includes silver-silica particle complexes; however, the suspension is unstable and is prone to gel at room temperature, requiring refrigeration during storage. Also, the process does not synthesize metal-particle complexes. Instead, it merely deposits metal nano-clusters on the surface of preexisting particles.
Recently, Kumar et al. have announced an in situ process for the formation of silver and gold nanoparticles in vegetable oil based paints using free radical reduction during the drying process. See Kumar et al., “Silver-nanoparticle-embedded antimicrobial paints based on vegetable oil,” Nature Materials, vol. 7, March (2008). While a promising route for synthesis of low cost coating containing silver nanoparticles, the process changes the color of the paint to a dark yellow (silver nanoparticles) or reddish color (gold nanoparticles), which is a problem that must be addressed before this technology may be used as an external, decorative paint. Also, Kumar et al. suggests that a proportion of the silver present in the coatings is in an ionized form, which may lead to leaching of the ionic silver form the coating.
Herein, nano-sized, nano-clusters and nanoparticles refers to a mean size (i.e. hydraulic diameter, cross-section or thickness) no greater than 100 nanometers and no less than one nanometer, and only refers to nanoparticles that are formed in a bottom-up synthesis.
The sulfur-containing functional additive 3-mercaptopropyl trimethoxysilane (MPTS) is used in sol-gel processing. However, 3-mercaptopropyl triethoxysilane (MPtriethoxysilane) is not known to be used in sol-gel processing, and one is not a functional equivalent of the other. Instead, the chemistry of each of these mercapto silanes is substantially different, with 3-mercaptopropyl-tri-ethoxysilane decomposing to ethanol on contact with water or humidity.